I. Field of the Invention
This invention relates generally to an electronic signal processing circuit for adapting a pyro/piezo sensor to a conventional polysomnograph machine of the type commonly used in sleep lab applications, and more particularly to an adapter that isolates pyro (breathing-related) signal components of the sensor from piezo (snore-related) signal components and which removes baseline noise from the piezo signal component.
II. Discussion of the Prior Art
In addressing sleep-related problems, such as sleep apnea, insomnia and other physiologic events or conditions occurring during sleep, various hospitals and clinics have established so-called sleep labs where through the use of instrumentation, a patient""s sleep patterns can be monitored and recorded for later analysis so that a proper diagnosis and therapy can be arrived at. A variety of sensors have been devised for providing recordable signals related to respiratory patterns during sleep. These sensors commonly are mechanical to electrical transducers that produce an electrical signal related to body movement. For example, in U.S. Pat. No. 5,331,968, the sensor comprises a plethysmograph band adapted to encircle a patient""s chest and which produces an electrical signal associated with chest movement due to respiratory activity. Other transducers include infrared pulse oximeters for monitoring blood oxygen levels, and microphones for detecting snoring. In addition, a plurality of electrodes for sensing myopotentials and/or cardiac rhythms are often utilized. These sensors feed their electrical signals into an electronics module referred to as a polysomnograph or PSG machine.
Recently, Dymedix Corporation, applicant""s assignee, has introduced a new type of sensor comprising a polyvinylidene fluoride (PVDF) film which is found to exhibit both pyroelectric and piezoelectric properties. Information relating to this type of sensor may be found in the Stasz U.S. Pat. 5,311,875 as well as in copending application Ser. No. 09/416,660, filed Oct. 12, 1999 (now U.S. Pat. No. 6,254,545). PVDF sensors of the type described are adapted to be affixed to a subject""s upper lip so that air flow due to inspiration and expiration impinge on the sensor to produce an output signal related to temperature changes occasioned by the inspiratory and expiratory flow. The sensor also is especially designed to pick up sound vibrations due to snoring episodes.
To successfully market this new type of sensor, it is desirable that it be able to be used with existing polysomnograph machines already in place in the sleep labs.
The present invention provides an adaptor for interfacing a pyro/piezo sensor to a PSG machine. It comprises a differential input amplifier having a pair of input terminals that are adapted to be coupled to the pyro/piezo sensor and an output terminal. The differential input amplifier is configured to significantly attenuate common-mode noise while providing a predetermined gain factor by which the sensor output signal is amplified. The output of the differential input amplifier is fed to a filter circuit that operates to separate a pyro component of the sensor output signal from a piezo component thereof The pyro signal component, which relates to inspiratory and expiratory air flow of the patient, feeds directly into a first channel of the PSG machine. The piezo component of the sensor output signal, which relates to sound or snoring, is first fed through a dead-band circuit which is effective to remove baseline noise from the piezo component of the sensor output signal. In coupling the resulting piezo component, absent its baseline noise, to the PSG sound channel, a differential output driver circuit is interposed.
By utilizing a differential input amplifier with a predetermined gain factor and by appropriately signal processing the amplified PVDF film sensor output signal, the pyro and piezo components thereof can be readily matched to existing PSG electronic boxes already on hand in most sleep labs.